They do have, and exercise common sense, and I don't doubt that for a moment.
Sometimes, I think, people may exercise poor judgement, that's no reflection on their ability to exercise common sense.
They choose to say or do things which may seem, to the outside observer, to be unwise, but what they choose to do is their choice.
Words on a screen aren't necessarily a reflection of the true character of their author.
Perhaps the grumpy, cynical, rude, sarcastic folk that you may encounter in internet land, from time to time, are simply people who are tired at the end of a hard day and just want to vent, to shout, to give voice to their frustations.You are "reading" people freed of the necessity of forced politeness that society imposes upon them. Their on line time is their time to be free, on line is where they get to shout and yell and let it all out.
Or maybe they are cynical, sarcastic, grumpy and rude. Who can say? 
Off line, in the real world, yes, I'm as human, as imperfect as anyone.
I have my cynical, grumpy and rude moments. It's a good thing that cars, with the windows wound up, are essentially sound proof. I've had some of my best swear-like-a-trooper angry rant sessions on bits of road between traffic lights. It's very therapeutic.
